Single Event Effects (SEE) are disturbances in an active semiconductor device caused by a single energetic particle. One type of SEE is a single event upset (SEU). SEU is a radiation-induced error in a semiconductor device caused when charged particles lose energy by ionizing the medium through which they pass, leaving behind a wake of electron-hole pairs, forming a parasitic conduction path. The parasitic conduction path causes a false transition on a node. The false transition, or glitch, propagates through the semiconductor device and ultimately results in the disturbance of a node containing state information, such as an output of a latch or register.
Typically, an SEU is caused by ionizing radiation components in the atmosphere, such as neutrons, protons, and heavy ions. The ionizing radiation components are abundant in space, even at commercial flight altitudes. Additionally, an SEU can be caused by alpha particles from the decay of trace concentrations of uranium and thorium present in some integrated circuit packaging. As another example, an SEU may be caused by detonating nuclear weapons. When a nuclear bomb is detonated, intense fluxes of gamma rays, x-rays, and other high energy particles are created.
Some semiconductor devices are designed to operate in conditions that expose the devices to energetic particles. These devices are typically modified to be hardened against SEE. For example, U.S. Pat. No. 6,753,694 describes obtaining SEU immunity by constructing a logic element with a redundant set of inputs and using two copies of each logic element to provide redundant outputs. When the redundant inputs agree, then the output of the logic element implements the logic function. However, when the redundant inputs disagree, then the output of the logic element is disconnected or tri-stated, which preserves the previous output value. U.S. Patent Application Serial. No. 2002/0175713 describes a similar method for hardening a device against SEE in which an output of the logic element is disconnected upon detection of an SEU. U.S. Pat. No. 6,753,694 and Patent Application Serial No. 2002/0175713 both rely on capacitance to store output data.
It would be beneficial to design a semiconductor device so that it can operate in conditions that expose the device to energetic particles and reliably store data. The present invention relies on a logical latching mechanism to store output data in addition to providing radiation hardness.